Electric power supply unit having improved output voltage response

ABSTRACT

The power supply unit is provided with a power supply circuit adapted to generate an output voltage in accord with an instruction voltage, an output condenser Co connected to the output end of the power supply circuit, and an auxiliary output voltage setting circuit adapted to compare the instruction voltage and the output voltage and to cause the output condenser to discharge its electric charge when the instruction voltage becomes lower than the output voltage. Because of the auxiliary output voltage setting circuit, the power supply circuit quickly generates an output voltage in accord with the instruction voltage if the instruction voltage is lowered.

This is a continuation of application Ser. No. 10/424,079 filed Apr. 25,2003, now U.S. Pat. No. 6,917,189 which application is herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to an electric power supply unit having improvedoutput voltage response to a change in instruction voltage.

BACKGROUND OF THE INVENTION

Switching power supply units have been widely used as power supply unitsfor efficiently converting a given supply voltage to a lower voltage. Inorder to smooth the intermittent output voltage arising from switching,a switching power supply unit has at the output end thereof a smoothingcoil and an output condenser.

The smoothing coil and the output condenser will delay the outputvoltage if a load is suddenly changed. Thus, there has been proposed animproved power supply unit adapted to alleviate the delay in theresponse to a change in load, as disclosed in, for example, JapanesePatent Early Publication H10-98874.

FIG. 1 shows a general circuit arrangement of a power supply unit havingan improvement in response to a delay pertinent to conventionalcircuits. A switching power supply circuit 1 of FIG. 1 performsswitching to convert a supply voltage Vin of a power source B such as abattery into a pulsed output having a fixed pulse width, based on thecomparison of the output voltage Vo with an internal reference voltage(not shown). The pulsating output is smoothed by a smoothing coil Lo andan output condenser Co, and supply a resultant output voltage Vo to aload 3. Diode D is a flywheel diode.

If this load 3 suddenly increases, the output voltage Vo is lowered. Inorder to compensate for the delay in recovery of the output voltage Voback to a predetermined level, a series power supply circuit 2 isconnected in parallel with the switching power supply circuit 1. Thus,when the output voltage Vo is suddenly lowered by a sudden increase ofthe load, a current is promptly supplied to the output condenser Co viathe series power supply circuit 2, thereby recovering the output voltageVo within a short delay time.

In this conventional electric power supply unit, the output voltage canbe recovered without an appreciable delay even if a rapid change takesplace in the load, provided that the power supply unit has a fixedreference voltage. However, in cases where the reference voltage islowered to change the output voltage, the conventional electric powersupply unit cannot quickly lower the output voltage because the seriespower supply circuit 2 has only a current-feeding capability.

FIG. 2 shows the reference voltage Vref, output voltage Vo, and loadcurrent Io changing in time during a transitional discharge. Suppose nowthat the reference voltage Vref is dropped from an initial level Vref1to a final lower voltage Vref2 at time t1, as shown in FIG. 2( a). Thenthe current Io flowing through the smoothing coil slowly decreases asshown in FIG. 2( c), because the discharge by the load 3 is slow andthat a back electromotive force is generated by the smoothing coil Lo.Consequently, the output voltage Vo sluggishly changes from its initiallevel Vo1 to a final level Vo2 associated with the reference voltageVref2 over a period T2, as shown in FIG. 2( b). It is noted that thewaveform of current Io shown in FIG. 2( c) corresponds to substantialswitching of the output transistor of the switching power supply unit.

It is, therefore, an object of the present invention to provide a powersupply unit having an output condenser, the unit adapted to change itsoutput voltage quickly to the target voltage defined by a variableoutput voltage setting.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided apower supply unit, comprising:

a power supply circuit adapted to compare an instruction voltage settinga target output voltage with the feedback voltage associated with theoutput voltage of said power supply unit to generate the target outputvoltage;

an output condenser connected to the output end of the power supplycircuit; and

an auxiliary output voltage setting circuit adapted to compare theinstruction voltage and the feedback voltage and, when said instructionvoltage is changed, to cause the output condenser to discharge itselectric charge as needed based on the comparison so that the outputvoltage quickly follows the changed instruction voltage.

In accordance with another aspect of the invention, there is provide apower supply unit for supplying an output voltage, comprising:

a switching power supply circuit having:

-   -   series output transistors connected between a power source        voltage and the ground;    -   a control unit for controlling the switching operations of the        output transistor based on the comparison of a feedback voltage        associated with the output voltage and the instruction voltage        for setting the level of the output voltage; and    -   a smoothing coil having one end connected to the node of said        series output transistors and another end outputting an        switching output;

an output condenser connected to the output end of the switching powersupply circuit; and

an auxiliary output voltage setting circuit adapted to compare theinstruction voltage and the feedback voltage and to cause the outputcondenser to discharge its electric charge as needed based on thecomparison, thereby permitting the output voltage to quickly follow thechanged instruction voltage.

In the electric power supply unit of the invention, if the instructionvoltage is altered to a lower voltage, the auxiliary output voltagesetting circuit causes the output condenser to discharge its charge sothat the resultant output voltage assumes the altered instructionvoltage. Accordingly, the output voltage will quickly become the voltageassociated with the altered instruction voltage.

It is noted that in this arrangement, the driving performance of theswitching power supply circuit is high when the output voltage is raisedhigh, and that the output voltage can be swiftly switched low when theoutput voltage is to be lowered, since then the output voltage ischanged on the output end of the smoothing coil by the auxiliary outputvoltage setting circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general circuit arrangement of a conventional power supplyunit.

FIG. 2 shows operational characteristics of a conventional power supplyunit.

FIG. 3 is a general circuit arrangement of a first embodiment of a powersupply unit according to the invention.

FIG. 4 shows operational characteristics of the power supply unit shownin FIG. 3.

FIG. 5 illustrates the effects of an offset voltage in the power supplyunit shown in FIG. 3.

FIG. 6 shows the circuit arrangement of an operational amplifier for usein a second embodiment of a power supply unit according to theinvention, in which the slewing rate of the operational amplifier isvariable.

FIG. 7 shows a circuit arrangement of a GM amplifier for use in theoperational amplifier FIG. 6.

FIG. 8 shows operational characteristics of the power supply unitutilizing the operational amplifier shown in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 is a general circuit arrangement of an electric power supply unitaccording to a first embodiment of the invention. FIGS. 4 and 5illustrate operations of the power supply unit.

As shown in FIG. 3, there is provided a switching DC/DC power supplycircuit section 10 having a switching circuit which includes n-type MOStransistors (hereinafter referred to as n-type transistors) Q1 and Q2connected in series between a source voltage Vin and the ground. Theswitching DC/DC power supply circuit section 10 also includes an erroramplifier OP2, an oscillator 11, a comparator Comp, and a driver 12. Theerror amplifier OP2 has a non-inverting input terminal (+) for receivingan instruction voltage Vdac and an inverting terminal (−) for receivingthe output voltage Vo of the power supply unit or a feedback voltageassociated with the output voltage Vo. The error amplifier OP2 providesat its output terminal an amplified difference between the two inputvoltages.

The oscillator 11 generates a frequency triangular output signal withthe high frequency (in the range from for example a few kHz to severaltens kHz). The comparator Comp has a non-inverting input terminal (+)for receiving the oscillatory output of the oscillator 11 and aninverting input terminal (−) for receiving the error output of the erroramplifier OP2, and supplies a resultant output to the driver 12. Basedon the comparison by the comparator Comp, the driver 12 alternatelyswitches on and off the n-type transistors Q1 and Q2 so that theiron-off statuses are in opposite phase.

The output voltage of the switching circuit provided at the node of theseries n-type transistors Q1 and Q2 is smoothed by the smoothing coil Loand output condenser Co. The charging voltage of the output condenser Cois supplied as the output voltage Vo to a load 30.

An output voltage instruction circuit 20 is composed of an operationalamplifier OP1, a resistor R1, and a variable resistor R2 whoseresistance is varied by a digital input signal Din. A constant referencevoltage Vref is supplied to the non-inverting input terminal of theoperational amplifier OP1. The instruction signal Vdac is changed bychanging the digital input signal Din to vary the resistance of theresistor R2.

An auxiliary output voltage setting circuit 40 is provided to allow theoutput condenser Co to discharge its electric charge. This auxiliaryoutput voltage setting circuit 40 is composed of an n-type transistor Q3connected in parallel with the output condenser Co, a comparator formedof an operational amplifier OP3 that provides an inverted output, and anoffset voltage source 41 having an offset voltage Voff. Thenon-inverting input terminal of the operational amplifier OP3 issupplied with the instruction voltage Vdac plus the offset voltage Voff,while the inverting input terminal is supplied with the output voltageVo, or the drain voltage of the n-type transistor Q3. The offset voltagesource 41 may be connected to the non-inverting input terminal if thepolarity of the offset voltage source 41 is reversed.

It should be understood that the power supply unit of the invention isnot limited to the one having a switching power supply circuit. Forexample, the invention encompasses a power supply unit having a powersupply circuit that is adapted to provide an output voltage defined byan instruction voltage input thereto by comparing the instructionvoltage for setting the level of the output voltage with a feedbackvoltage associated with the output voltage.

Referring to FIGS. 4 and 5, operations of the power supply unit of theinvention as shown in FIG. 3 will now be described below. Upon receiptof an arbitrary instruction voltage Vdac supplied from the outputvoltage instruction circuit 20, the error amplifier OP2 of the switchingDC/DC power supply circuit section 10 calculates the difference betweenthe instruction voltage Vdac and the output voltage Vo. In accordancewith the difference between the output of the error amplifier OP2 (theoutput referred to as error output) and the output of the oscillator 11(the output referred to as oscillator output), the driver 12 controlson-off operations (referred to as switching control) of the n-typetransistors Q1 and Q2 of the switching circuit. As a result, theresultant output voltage Vo in accord with the instruction voltage Vdacis supplied to the load 30. Thus, while the instruction voltage Vdacremains constant, the switching control is performed for the outputvoltage, supplying a constant output voltage to the load if the loadchanges.

However, there can be occasions where the instruction voltage Vdac needsto be varied in response to a request of the load. If in such a case theinstruction voltage Vdac is to be changed to a higher voltage, the powersupply circuit can provide a sufficient current to charge the outputcondenser Co, so long as the source voltage Vin is higher than theoutput voltage Vo, so that the output voltage Vo can be varied ratherquickly in accord with the new instruction voltage. The output voltagecan be varied to a higher voltage even faster by providing a seriespower supply circuit as is conventional.

However, it is not the case when the instruction voltage Vdac is to bechanged to a lower voltage. Ordinarily, the current Io is flowing in thedirection as indicated by an arrow as shown in FIG. 3. In order to lowerthe output voltage Vo in accord with the instruction voltage Vdac, it isnecessary to have the output condenser Co discharge its electric charge.

The charge can be discharged via the load 30 and via the smoothing coilLo and n-type transistor Q2. However, either of the two discharge routeshave only a limited discharge capability, so that the output voltage canlower only slowly.

The auxiliary output voltage setting circuit 40 of invention enables aquick change in the output voltage Vo in response to the instructionvoltage Vdac changing from a high to a lower voltage.

More particularly, at the moment when the instruction voltage Vdac ischanged from a given voltage Vdac1 to a new lower voltage Vdac2, theoutput voltage Vo1 becomes higher than the sum of the instructionvoltage Vdac2 and the offset voltage Voff (Vdac2+Voff). Consequently,the output voltage of the inverting type operational amplifier OP3 goesto a HIGH level, turning on the n-type transistor Q3. This turning on ofthe n-type transistor Q3 causes the output condenser Co to dischargequickly, resulting in a quick reduction of the output voltage Vo.

This operation will be further described in detail with additionalreference to FIG. 4. Suppose that the instruction voltage Vdac ischanged from a higher voltage Vdac1 to a lower voltage Vdac2 at time t1,as shown in FIG. 4( a). At this moment, the n-type transistor Q3 ispromptly turned on. Since the on-resistance of the n-type transistor Q3is extremely small as compared with the load 30 at the moment the n-typetransistor Q3 is turned on, the current Io indicated in FIG. 3 flowsmomentarily in the opposite direction, as shown in FIG. 4( c). It isnoted that there is not anything like the smoothing coil Lo that hindersthe current, so that the impedance of the path is extremely small ascompared to the impedance of the load. Hence, the charge of the outputcondenser Co is quickly discharged.

The output voltage of the operational amplifier OP3 returns to the LOWlevel when the output voltage Vo becomes equal to the sum (Vdac2+Voff).Then, the n-type transistor Q3 is turned off, and the discharge of theoutput condenser Co terminates.

Accordingly, the output voltage Vo is changed from the pre-instructionoutput voltage Vo1 to the post-instruction output voltage Vo2 asdetermined by the reduced instruction voltage Vdac2 in a short period oftime T1, which is one n^(th) of conventional time T2 (T1<<T2) with nbeing in the range between 1 and 100.

The offset voltage Voff is a minute voltage to compensate for theso-called under-shoot of the output voltage caused by the delayingresponse of operational amplifier OP3 and the n-type transistor Q3.

Without the offset voltage Voff, the output voltage Vo would once overlydrops, or undershoot, below a target voltage Vo2, caused by operationaldelays of the operational amplifier OP3 and the n-type transistor Q3, asshown in FIG. 5( b). Then, it would take some time for the outputvoltage Vo to come back to the target voltage Vo2, requiring a longertime T1 u, say, than T1 (T1<T1 u<T2).

In order to overcome this problem, the offset voltage source 41 sets upan offset voltage Voff to take account of operational delays of theoperational amplifier OP3 and the n-type transistor Q3. With a properoffset voltage Voff, the undershoot is circumvented and the targetoutput voltage Vo2 in accord with the changed instruction voltage Vdac2results in an extremely short period of time T1, as shown in FIGS. 4( b)and 5(a).

Next, referring to FIG. 6, there is shown an arrangement of anoperational amplifier OP3 that allows for a variable slewing rate inaccordance with a second embodiment of the invention. Slewing rate isdefined to be a magnitude of rise or fall in the output per unit timefor a given step-up or step-down change in the input. FIG. 7 shows acircuit diagram of a mutual conductance amplifier for use as theoperational amplifier shown in FIG. 6. FIG. 8 is a graphicalrepresentation of the operations of the power supply unit utilizing theoperational amplifier shown in FIG. 6.

As shown in FIG. 6, the operational amplifier OP3 has a differenceamplifier 42 and a GM amplifier 44. The difference amplifier 42 issupplied with a summed voltage (Vdac+Voff) and the output voltage Vo,and amplifies the difference voltage between them.

The difference amplifier of FIG. 6 includes a first series branchconsisting of a resistor R3 and an npn-type bipolar transistor(hereinafter referred to as npn transistor) Q4, a second series branch,connected in parallel with the first series branch, consisting of aresistor R4 and an npn transistor Q5, and a constant current source 43connected in series between the first and the second series branches andthe ground for supplying a constant current Icom. The base of the npntransistor Q4 is supplied with the summed voltage (Vdac+Voff), while thebase of the npn transistor Q5 is supplied with the output voltage Vo.

The GM amplifier 44 is supplied with both the summed voltage (Vdac+Voff)and the output voltage Vo to output a current Ig which is proportionalto the difference voltage |(Vdac+Voff)−Vo| between them. The current Igacts to apparently increase the constant current Icom of the differenceamplifier 42. Thus, the slewing rate is changed by inputting the voltagedifference |(Vdac+Voff)−Vo| to vary the current supplying performance ofthe operational amplifier OP3.

An exemplary configuration of such GM amplifier 44 is shown in FIG. 7.In the example shown herein, the GM amplifier 44 is composed of constantcurrent sources I51, I52, I53, and I54, npn transistors Q51, Q52, Q57,Q58, Q59, and Q60, pnp transistors Q53, Q54, Q55, and Q56, and aresistance R51, as shown.

In the GM amplifier as shown in FIG. 7, larger one of the summed voltage(Vdac+Voff) and the output voltage Vo is generated at one end of theresistor R51 (the end connected to the emitter of the npn transistorQ51), and smaller one is generated at the other end of resistance R51.As a consequence, the current Ig becomes substantially proportional tothe magnitude of the difference voltage |(Vdac+Voff)−Vo| divided by theresistance of the resistor R51.

Operations of this electric power supply unit utilizing the operationalamplifier OP3 having this variable slewing rate will be described withreference to FIG. 8. It should be understood that the offset voltageVoff is omitted in FIG. 8 for simplicity.

Suppose now that the power supply unit is currently in first statusoutputting a first (high) voltage Vo1 in accord with a first (high)instruction voltage Vdac1 until the instruction voltage Vdac is changedto a second (lower) instruction voltage Vdac2, as shown in FIG. 8( a).In this instance, the difference (Vdac2−Vo1) in voltage between thefirst output voltage Vo1 at the moment of the change and the changed(i.e. second) instruction voltage Vdac2 is amplified by the differenceamplifier 42.

It is noted that the difference amplifier 42 is operated by the sum ofthe constant current Icom and the current Ig of the GM amplifier 44associated with the difference voltage supplied to the differenceamplifier 42. When the difference voltage (Vdac2−Vo1) is small, a smallcurrent Ig is added, thereby rendering the circuit to operate at a lowslewing rate. In this case, the output voltage Vo changes from the firstoutput voltage Vo to the second output voltage Vo2 after certain timeT3, as shown in FIG. 8( b).

Next, consider a case where the instruction voltage Vdac is changed to afurther lower third instruction voltage Vdac3 (i.e. Vdac3<Vdac2), asshown in FIG. 8( a). In this case also, the difference voltage(Vdac3−Vo1) between the third instruction voltage Vdac3 and the firstoutput voltage Vo1 at the time of the change is amplified by thedifference amplifier 42.

This time, the current Ig is increased, before it is added to theconstant current Icom in accordance with an increase in the differencevoltage supplied to the difference amplifier 42, thereby rendering theoperational amplifier OP3 to operate at a large current. Hence, theoperational amplifier OP3 is operated at a high slewing rate inaccordance with a large difference voltage (Vdac3−Vo1). Thus, if achange in the instruction voltage is large, the output voltage Vochanges from the first output voltage Vo1 to the third output voltageVo3 in substantially the same time interval T3 as shown in FIG. 8( b).

In this way, by adding an GM amplifier 44 to the difference amplifier42, the slewing rate of operational amplifier OP3 increases with thedifference of the two inputs thereof, thereby prompting the rise of theoutput of the operational amplifier OP3 and hence discharge of theoutput condenser Co.

1. A power supply unit comprising: a power supply circuit adapted tocompare a predetermined voltage with a detection voltage associated withsaid output voltage so as to generate said output voltage in accord withsaid predetermined voltage; an auxiliary output voltage setting circuitadapted to compare a voltage associated with said predetermined voltagewith said detection voltage and, based on said comparison, discharge theelectric charge stored in an output condenser connected to the outputend of said power supply circuit, wherein said auxiliary output voltagesetting circuit comprises: a comparator for comparing a voltageassociated with said predetermined voltage and said detection voltagethat are received as two inputs; and a switch connected in parallel withsaid output condenser and turned on by the output of said comparator,causing said output condenser to discharge.
 2. The power supply unitaccording to claim 1, wherein an offset voltage is added to either oneof said two inputs such that the input terminal receiving saidpredetermined voltage has a higher voltage than the other inputterminal.
 3. The power supply unit according to claim 2, wherein theslewing rate of said comparator increases with the difference betweensaid two inputs.
 4. The power supply unit according to claim 3, whereinsaid comparator has a current driven error amplifier for amplifying thedifference between said two inputs; and a mutual conductance amplifierfor increasing the operational current of said difference amplifier inaccordance with the difference between said two inputs.
 5. The powersupply unit according to claim 1, wherein the slewing rate of saidcomparator increases with the difference between said two inputs.
 6. Thepower supply unit according to claim 5, wherein said comparator has acurrent-driven error amplifier for amplifying the difference betweensaid two inputs; and a mutual conductance amplifier for increasing theoperational current of said difference amplifier in accordance with thedifference between said two inputs.
 7. A power supply unit, comprising:a switching power supply circuit having series output transistorsconnected between a power source voltage and the ground, and a controlunit for controlling the switching operations of said output transistorsbased on the comparison of a predetermined voltage for setting the levelof the output voltage of said power supply unit with a detection voltageassociated with said output voltage, and a smoothing coil having one endconnected to one node of said series output transistors and another endfor outputting the switching output; and an auxiliary output voltagesetting circuit adapted to compare a voltage associated with saidpredetermined voltage with said detection voltage and, based on saidcomparison, cause an output condenser connected to the output end ofsaid switching power supply circuit to discharge its electric charge. 8.The power supply unit according to claim 7, wherein said auxiliaryoutput voltage setting circuit comprises: a comparator for comparing avoltage associated with said predetermined voltage and said detectionvoltage that are received as two inputs; and a switch connected inparallel with said output condenser and turned on by the output of saidcomparator, causing said output condenser to discharge.
 9. The powersupply unit according to claim 8, wherein an offset voltage is added toeither one of said two inputs such that the input terminal receivingsaid predetermined voltage has a higher voltage than the other inputterminal.
 10. The power supply unit according to claim 9, wherein theslewing rate of said comparator increases with the difference betweensaid two inputs.
 11. The power supply unit according to claim 10,wherein said comparator has a current-driven error amplifier foramplifying the difference between said two inputs; and a mutualconductance amplifier for increasing the operational current of saiddifference amplifier in accordance with the difference between said twoinputs.
 12. The power supply unit according to claim 8, wherein theslewing rate of said comparator increases with the difference betweensaid two inputs.
 13. The power supply unit according to claim 12,wherein said comparator has a current driven error amplifier foramplifying the difference between said two inputs; and a mutualconductance amplifier for increasing the operational current of saiddifference amplifier in accordance with the difference between said twoinputs.